A computer graphic input device that uses a prism to provide the input sensitive drawing surface has been described by Richard Greene in U.S. Pat. No. 4,561,017, the disclosure of which is incorporated by reference. An improved version of that device that adds a collimating lens to provide uniform sensitivity and no keystone distortion has been described by Richard Greene in U.S. Pat. No. 5,181,108, the disclosure of which is also incorporated by reference. Both devices use conventional prisms and the latter device uses conventional collimators, each of which has continuous surfaces and therefore requires a great deal of transparent material, is expensive to manufacture, and is heavy in weight, for all but the smallest drawing surfaces.
Those devices also required specialized optics or cameras to correct for rectangular distortion that remained in the image, which are also expensive.